The present invention refers in general to eddy current sensor assemblies in which a position of an encoder element is determined relative to a coil assembly on the basis of position-dependent eddy-current losses, and the present invention refers particularly to an encoder element for such a sensor assembly.
In many technical fields the position of a moving object must be determined with an accuracy dictated by the respective purposes of use. To this end many sensor systems have been developed in which at least the relative position between two elements can be measured with a sufficiently high precision by e.g. optical, electrical, magnetic and other interactions. Particularly in technical sectors in which very exacting ambient conditions prevail, e.g. high operating temperatures in combination with high magnetic fields, which may e.g. be caused by high operating currents, sensor assemblies are often used in which the position-dependent generation of eddy currents is used for determining the position of a component. To this end in some examples of such an eddy-current sensor assembly, the attenuation of one or a plurality of coils, which is caused by eddy currents, is sensed, wherein the one or a plurality of coils is provided as stationary components, and a moved component comprises a track of a suitable material, resulting in a position-dependent eddy-current generation and thus attenuation. On the basis of said position-dependent eddy current generation, through correlation of the generated attenuation, which can e.g. be determined on the basis of a frequency change, phase change, amplitude change or a combination of said parameters, and the particular design of the track, it is possible to determine the position of the moving track in relation to the one or more stationary coils.
An important application in this respect is the determination of the position of a rotor of an electric machine to determine suitable control signals for the supply of suitable current and voltage values. For instance, it is important for many application purposes where very variable rotational speeds and a moderately large control bandwidth are needed for the electric machine that the voltage or current values are injected with a high resolution in time, which requires a relatively precise determination of the position of the rotor in many cases. For instance, it is important for an efficient operation of approximately permanently excited synchronous machines or also brushless DC machines that the position of the rotor is quite precisely known in conformity with the used pole number of the magnets inside an angular segment to as to energize the stator windings with a suitable pattern, whereby the desired mode of operation is obtained. Contactless sensor mechanisms are often used for this purpose, wherein sensor assemblies based e.g. on magnetic switches or Hall sensors are used, which are, however, relatively complicated and also susceptible to noise, wherein the necessary spatial resolution is often not reached for many applications as a rule. Since particularly with applications for detecting the angular position a spatially narrow coupling of the encoder element with the rotor of the electric machine is often required, high currents or magnetic fields and also relatively high temperatures may occur near the encoder element, so that sensors according to the eddy current principles have here turned out to be particularly useful as they can be designed on the one hand such that they are not significantly interfered with by high external magnetic fields and permit, on the other hand, a high spatial resolution of the rotor position also at the time when the sensor assembly is activated.
In such conventional sensor assemblies an encoder wheel is typically used that is e.g. coupled with the rotor of the electric machine and the periphery of which comprises an appropriately formed track, e.g. in the form of a sinusoidally changing metal conductor, e.g. of copper or aluminum, which is read out by a coil assembly. This encoder wheel is thus in mechanical contact with the rotor of the electric machine and, apart from the desired noise immunity with respect to high magnetic fields, it must also be adapted to the other operating conditions of the electric machine, e.g. with respect to the prevailing temperatures, the necessary speed range, or the like. On the other hand, with respect to the integration of the eddy-current sensor system a high constant precision is also needed in mass manufacture for permitting a uniform function of the sensors without any complicated adjusting work during installation for end use. As a rule, in conventional encoder wheels, the exacting mechanical properties with respect to strength, quiet running, and the necessary electrical properties, i.e. the position dependent adjustment of conductivity by means of an approximately sinusoidally changing coil, therefore require corresponding efforts in the manufacture of the encoder wheels, their final assembly and adjustment.